The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with medical diagnostic magnetic resonance imaging and will be described with particular reference thereto. However, it is to be appreciated that the present invention also finds application in magnetic resonance spectroscopy and magnetic resonance imaging for other applications.
Generally, nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) techniques employ a spatially uniform and temporally constant main magnetic field, B.sub.0, generated through an examination region. Superimposed on the B.sub.0 magnetic field is a B.sub.1 radio frequency (RF) magnetic field at the NMR resonant frequency. For MRI applications, there is also a set of gradient magnetic fields used to spatially encode resonant spins. Some MRI techniques such as fat suppression are extremely sensitive to magnetic field homogeneity at the one part per million level. The geometric shape and/or magnetic susceptibility of a subject being scanned can induce local non-uniformities in the magnetic fields as high as 1 to 3 ppm. Non-uniformities of this strength are large enough to produce local imaging and fat suppression problems.
Fat suppression is generally a known imaging technique wherein the relative brightness of fat and water in an image is changed to better reveal diagnostic information. The resonant frequency of fat is about 3.5 ppm below that of water. Consequently, one can selectively suppress either water, or more commonly, fat on the basis of the resonant frequency difference. Typically, this is accomplished by selectively exciting fat with a narrow band RF pulse including frequencies near the characteristic resonance frequency of fat. In principle, this leaves the water alone. Ideally, the radio frequency pulse is applied with a maximally uniform B.sub.0 magnetic field. The magnetization that is not properly aligned is then spoiled with the application of a magnetic gradient following the RF pulse. The magnetization of fat is not given time to recover before the imaging sequence is run; thus, the fat signal contribution to the image is suppressed. However, if the magnetic field B.sub.0 is not spatially constant, fat may not be uniformly suppressed or regions of water may be suppressed. Due to the shape of the RF pulses employed and the strength of the fat signal, magnetic field changes of less than 1 ppm can be seen as non-uniform fat suppression.
Because of geometric shape and magnetic susceptibility factors, there are some subjects and/or parts of the anatomy where fat suppression is particularly problematic. For example, the anatomy of the face and ankle are two regions where these factors affect local magnetic field homogeneity. Another one of the more problematic regions for scanning with the fat suppression technique is the cervical spine. Due to geometric shape and susceptibility factors, the fat in the shoulder to neck transition region can have a magnetic field 2 to 3 ppm higher than the magnetic field was at that same position without the subject present.
Previous methods for controlling the homogeneity of the magnetic field include both passive and active shimming techniques. The passive technique is typified by arranging shim steel to minimize static magnetic field inhomogeneities base upon NMR field plot measurements. Generally these steel shims are placed at a relatively large distance from the region of interest. For example, in cylindrical-type MRI apparatus, the steel shims are commonly placed at diameters comparable to the gradient coils and/or whole-body RF coils. The NMR field plot measurements are performed without a subject in the examination region. Generally, the shim steel technique is not adjustable on a scan-by-scan basis. It is mainly used to shim out the effects of magnet-built tolerances and environmental (site) effects. Therefore, this technique is not suited to handle local non-uniformities within the magnetic field caused by subject geometry and/or susceptibility.
Active shimming generally employed multiple orthogonal shim coils and/or gradient coil offsets. Shim and/or gradient offsets were adjusted at the beginning of each scan, especially before fat suppression, to optimize the B.sub.0 uniformity with the subject present to account for susceptibility effects. The procedure generally looked to maximize the water signal while minimizing its spectral line width. In some cases, initial optimal shim currents were applied to the shim coils to initially establish uniform magnetic fields using the same type of NMR field plot measurements described above with reference to the passive technique. Commonly first order and occasionally second or third order corrections were implemented to compensate for non-uniformities in the magnetic field. While better uniformity was achieved using the active shimming techniques, the performance remained inadequate for certain applications. That is to say, some susceptibility based magnetic field uniformity problems occur over short ranges and/or small local regions of interest and were heretofore too difficult to shim out with lower order corrections.
Water and/or fat suppressed images may be achieved in a number of manners. One technique employs a spatial/spectral RF pulse for a 90.degree. imaging pulse. This approach does not modify the pulse sequence to include a fat saturation pulse at the start. Rather, a water selective 90.degree. pulse is built into the sequence from the start. However, this technique suffers the same magnetic field homogeneity problems as the fat suppression technique.
Another alternative to fat suppression is an inversion recovery sequence. Fat has a shorter T2 vector decay than water. In an inversion recovery sequence, a 180.degree. RF pulse is applied, and both the water and fat magnetization is inverted. After 140 ms on a 1.5 T machine, the fat signal has dissipated, but there is remaining water to excite in the transverse plane for an MRI signal. This water signal can be used to produce images; however, they may not necessarily be the fast images a physician may desire.
As well, there are water and fat separation images. These require multiple acquisitions, more time, and more processing. Usually, these are based on the different resonant frequencies of fat and water causing the imaging signal from each voxel to move in and out of phase as time passes. Two images are calculated for each image element from the data, and the more quickly decaying signal is assigned to the fat image. One disadvantage is the inflexibility of the sequence and effective time interval, TE, from the RF pulse to the measurement of the MRI signal.
The present invention contemplates a new and improved localized shim coil for use in magnetic resonance applications which overcomes the above-referenced problems and others.